Open Magnetic Fields in the Martian Magnetosphere Revealing Dipole-like Intrinsic Magnetic Fields at Mars

Mars’s magnetosphere is hybrid, having contributions from both an induced magnetosphere like Venus and the localized crustal magnetic fields. However, the planetary fields also include large-scale, more global components. In this study, we investigate their role in Mars’s magnetospheric topological responses to the interplanetary magnetic field (IMF) clock angle using observations from the Mars Atmospheric Volatile and EvolutioN mission. We show that the large-scale planetary field has a “dipole-like” influence on the Mars global magnetosphere by examining the open field topology. We find that the “dipole-like” planetary field, as at Earth, results in a more open magnetosphere during southward IMF. The clock angle effects on the twisted magnetotail current sheet are similarly consistent with this analogy. It reinforces the idea that Mars’s magnetosphere and solar wind interaction are more Earth-like than previously thought.


Introduction
The Martian magnetosphere is hybrid and unique in the solar system in that it has features of both the "induced" magnetosphere of Venus, consisting of the external/interplanetary magnetic field (IMF) wrapping around the planetary (Luhmann 1986;Phillips & Russell 1987;Zhang et al. 2010), but it has also been modified by the localized interaction with its crustal magnetic fields (Acuna et al. 1999;Connerney et al. 2005).However, in spite of this broad understanding, the largescale magnetic field geometry of the Martian magnetosphere is not readily visualized as it is for other planetary magnetospheres where global dipole fields are in control.The purpose of the present study is to provide further insight into the characteristics of the Martian magnetosphere, toward both improving the analyses of observations that allow us to infer physical drivers, and better placing Mars in the context of solar system magnetospheres.
The two dominant IMF orientations, associated with the outward and inward "Parker Spiral" sectors of the heliospheric magnetic field, produce two prevailing magnetic polarity patterns in statistical cross sections of the average magnetotail field.For a purely induced magnetosphere as Venus, the cross sections generally exhibit outward and inward field (with respect to the planet) "halves" in a roughly east-west pattern, with a tail current sheet perpendicular to the upstream IMF direction separating two opposite-pointing magnetotail lobes (e.g., Luhmann 1986;Zhang et al. 2010).At Mars, however, this tail current sheet is warped in between the two loosely twisted magnetotail lobes (DiBraccio et al. 2018(DiBraccio et al. , 2022)).
These twisted magnetotail field polarity cross sections bear a resemblance to those expected for a dipolar planetary field magnetosphere produced by an axially aligned weak planetary dipole (e.g., Axford 1991;Dubinin et al. 1994).Models of the solar wind interaction with Mars suggest the sense and degree of the observed twist are related to its crustal magnetic fields (Connerney et al. 2005).The crustal fields could act as an effective global dipole moment at high altitudes that becomes significant in the region of the dayside solar wind interaction boundary.In reality, the crustal field-dominated Martian magnetosphere topology is constantly reconfigured by the planet's rotation (Ma et al. 2014), but the high-order planetary fields have relatively modest influences on the tail cross sections by ∼2 Rm downstream (DiBraccio et al. 2022).It is worth noting here that the relatively strong dipolar field magnetosphere of Earth exhibits a similar version of this IMFdependent "twist" in magnetotail cross sections.In this case, the twist's origins have been attributed to the draped magnetosheath field/planetary dipole field reconnection patterns on the dayside magnetopause asymmetrically supplying new "open" flux to the tail lobes (Fedder et al. 1995).
More recently, Dubinin et al. (2023) characterize the measured magnetic fields in different coordinates to disentangle the induced and intrinsic components of Mars's hybrid magnetosphere, promoting the induced component in the Mars Solar Electric coordinate system and observing the effects of the crustal fields in the Mars Solar Orbital (MSO) coordinate system.Dubinin et al. (2023) suggest that the global effect caused by magnetic reconnection between the crustal magnetic field and the IMF is similar to that of a global dipole, which can help explain the magnetotail twist.Meanwhile, Bowers et al. (2023) reveal that some of the crustal magnetic fields have the highest magnetic shear with the southward IMF (SIMF) compared to other IMF clock angles.This provides another support that, despite appearing highly structured, the Mars crustal magnetic fields have a favorable orientation prone to more magnetic reconnection occurring under the SIMF.
In this study, we utilize an entirely different tool, magnetic topology, to isolate the purely induced and intrinsic components of Mars's magnetosphere, represented by the draped topology with both ends connected back to the solar wind (no connection to the Mars ionosphere) and the open topology (with one end connected to the Mars ionosphere and the other end to the solar wind), respectively (Xu et al. 2020).Note that magnetic topology is defined with respect to the superthermal electron exobase (∼160 km altitude) rather than the surface (Xu et al. 2019b), such that the inferred open field lines can be either connected to the surface or deeply draped (i.e., have a draped topology but penetrate below the electron exobase).Therefore, in this study, the induced component is defined as the ideal draping of the IMF around the planet as in the case of Venus, while the intrinsic component is defined as the resulting "open" field lines from the Sun-Mars interaction that cannot be canceled by averaging all IMF directions.More specifically, we examine the response of these two topologies to the interplanetary field inclination or "clock angle."We then examine the field polarities of open magnetic fields at low altitudes and in the tail, which reveals a dipole-like intrinsic magnetosphere.This study provides a better understanding of the nature of the observed Martian magnetospheric fields, both the induced component and the intrinsic component.

Open Field Topology at Low Altitudes
We first examine the occurrence rate of the open topology as a function of the IMF clock angle with Mars Atmospheric Volatile and EvolutioN (MAVEN) observations.Observationally, magnetic topology is determined based on the superthermal (>1 eV) electron energy and pitch angle distributions measured by the Solar Wind Electron Analyzer (SWEA) instrument (Mitchell et al. 2016) on board MAVEN (Jakosky et al. 2015) using a technique described by Xu et al. (2019b).More specifically, the presence of loss cones in the electron pitch angle distributions implies a magnetic connectivity to the nightside collisional atmosphere (Brain et al. 2007;Weber et al. 2017) and the presence of ionospheric photoelectrons (identified from electron energy distributions) indicates a magnetic connectivity to the dayside collisional ionosphere (Xu et al. 2016(Xu et al. , 2017)).The two topologies discussed in this study are open and draped.An open field line that connects the dayside or nightside ionosphere on one end and the solar wind on the other end would be identified by outflowing photoelectrons and in-flowing solar wind electrons ("open-day") or one-sided loss cone pitch angle distributions ("open-night").The draped topology is inferred when no loss cone is observed in pitch angle distributions and no photoelectron is identified in either field-aligned direction.The upstream IMF is determined by the measured magnetic vectors by the Magnetometer (Connerney et al. 2015) when MAVEN samples the upstream solar wind (Halekas et al. 2017) but is otherwise approximated by a proxy provided by Ruhunusiri et al. (2018).Upstream drivers are orbit averages given at apoapsis times and linearly interpolated to local measurements based on time.The data used in this study are MAVEN observations from 2014 December to 2020 March.
Figure 1  ) in the MSO coordinates in panels (a)-(c) inferred from the MAVEN observation.All solar zenith angles are included but the open-day topology (red lines) should be mostly on the dayside and near the terminator and the open-night topology (blue lines) on the nightside.In MSO, the X-axis points from the center of Mars to the Sun, the Z-axis points to the ecliptic north pole of Mars's orbital plane, and the Y-axis completes the right-hand system.All three panels show that there are more open field lines (black lines) during the SIMF and that this preference of the SIMF is mainly caused by open field lines connected to the dayside, particularly at high altitudes (>300 km), probably a result of dayside magnetic reconnections (Harada et al. 2018(Harada et al. , 2020)).
To further understand the SIMF preference, Figures 1(d)-(f) display the measured local B r /|B| of open fields at 200-300 km altitude (close to the footpoints of the open fields) in the geographic coordinates, for all subsolar longitudes, southern strong crustal fields on the dayside and on the nightside, respectively.Note that the geographic coordinates are fixed to the Mars body and are used here to highlight the importance of crustal field locations, which are smeared out in both latitude and longitude the MSO coordinates.All three panels show similar patterns, mostly radially outward magnetic fields (blue) in the south and radially inward magnetic fields (red) in the north, ignoring the fine structures, similar to what was measured by Mars Global Surveyor at 400 km altitude and 2 AM LT (local time; e.g., Figure 11 of Brain et al. 2006 and also (Mittelholz et al. 2017)).Such a global pattern resembles a southward-pointing dipole field moment having field lines going from south to north, which would favor the SIMF to produce more open fields.

Tail Morphology Separated for Magnetic Topologies
We next examine the tail-lobe morphology separately for open and draped topologies (main topologies in the tail) in the MSO coordinate system.Figure 2 illustrates the tail polarity (B x /|B|) for all topologies (top row), open fields (middle row), and draped fields (bottom row) in the MSO Y-Z plane for ).In comparison, the tail lobes for draped fields (A3)-(B3) are more induced-like (Venus-like) with a current sheet mainly along Z MSO , with arguably similar tail twists manifested at L MSO < 1 R M (to be discussed later).
When combining all upstream IMF conditions (panels (C1)-(C3) in Figure 2 Similar magnetic morphologies can be seen at different X MSO ranges.Figure 3

Synthesis
We use schematics in Figure 4 to synthesize results from Figures 1-3.As illustrated in Figure 4(A1), a southward dipole field generates closed magnetic field lines going outward from the southern hemisphere, turning northward and then inward in the northern hemisphere.This dipole field would preferentially magnetically reconnect with the SIMF on the dayside, similar to the preference of SIMF shown in Figure 1.The open field lines resulting from this interaction, as shown in Figure 4(A2), point sunward in the north and tailward in the south on both dayside and nightside.The tail-lobe configuration in the left panel resembles the tail polarities of open fields observed by MAVEN when combining all IMFs (Figures 2-3).In the case of Mars, the dipole-like lobe configuration is, however, unlikely to have a significant contribution from the very weak dipole term of the crustal fields from spherical harmonics models, but rather may be a global effect of Mars's intrinsic fields.
The north-sunward-south-tailward tail lobes resulting from the dipole-like intrinsic field can also help explain the tail twist.Take the case of the east IMF as an example.As illustrated in Figures 4(B1)-(B4), when we divide the tail lobes into four quadrants from both the dipole component (panel (B1)) and the IMF draping (panel (B2)), quadrants 2 and 4 have the same orientation from both components while quadrants 1 and 3 have the opposite orientations.The opposite-oriented open and draped fields in quadrants 1 and 3 enable more magnetic reconnection between the two (dashed lines in Figure 4(B3)).This reconnection produces new open field lines, shown as the solid lines in Figure 4(B3), that are now connected to the IMF on the opposite side of the planet back to the solar wind.When these new open fields relax and straighten out, it adds a magnetic torque to quadrants 1 and 3 and causes them to twist clockwise, resulting in a rotated current sheet as illustrated in panel (B4).A similar mechanism was evoked to explain the tail twist at Earth associated with the Y-component of the IMF (Cowley 1981).The case of the west IMF works in a similar fashion and we thus forgo the discussion.It is worth noting that a weak dipole-like component is also present in draped fields when averaged over all IMF directions (Figures 3(e)-(f)), which might be because the interaction between the draped IMF and tail open fields has a feedback to the draped IMF as well and/or the possible errors in the upstream IMF estimates.

Possible Origins of the Large-scale Dipole-like Field
As demonstrated in DiBraccio et al. (2018), there is no tail twist in the MHD simulation for the noncrustal field case (Venus-like), making the observed tail twist at Mars most likely a result of its intrinsic fields.Dubinin et al. (2023) suggest that the global effect caused by magnetic reconnection between the crustal magnetic field and the IMF is similar to that of a global dipole.How the Mars intrinsic fields might provide this global dipole-like field effect is an intriguing and important question.
Here, we reexamine the measured polarities near the footpoints of open field lines (Figures the strength of the crustal fields varies by orders of magnitude at different geographic locations, similar to Figure 9(a) in Bowers et al. (2023), we can divide them into three regions for an easier discussion: the strongest southern crustal field regions (Lat < ∼−30°and 120°< Long < 240°; SCR), the moderate crustal field belts near the geographic equator (MCR), and the weak crustal field regions (WCR).Unlike the SCR, both the MCR and WCR show an overall north-inward-south-outward configuration (Figures 1(d)-(f)).Bowers et al. (2023) suggest that both the MCR and WCR have the highest magnetic shear with SIMF compared to other IMF clock angles.A higher magnetic shear means a higher probability of magnetic reconnection between the crustal fields and the IMF, leading to more open field lines.However, we note that it does not require SIMF to generate such a north-sunward-south-tailward lobe configuration.Instead, it applies to all IMF directions because of the magnetic polarities of the MCR and WCR themselves.That is, the intrinsic fields can magnetically reconnect with any IMF orientations that are present, or even go through multiple reconnections, but must to be radially inward in the north and outward in the south near the footpoints of the resulting open fields.It is worthwhile to discuss the possible contributions of the MCR and WCR separately.For the MCR, it is most likely a configuration of a belt of sizable (tens of degrees of latitude) dipoles (instead of a global dipole) with closed loops going out from south and then into the north, spanning over almost all geographic longitudes near the geographic equator.It is interesting that while these moderate crustal fields appear randomly oriented, they actually form a coherent south-to-north orientation at higher altitudes, which might have implications for the formation mechanism of Mars's crustal magnetic fields.In terms of the global effects, open field lines rooted at MCR are spatially confined near the footpoints but should expand to larger magnetic flux tubes with increasing altitudes as the field strength decreases, probably occupying a significant fraction of the magnetotail.
Meanwhile, the WCR is generally considered to be dominated by induced magnetic fields in the ionosphere, which are stronger on the dayside than on the nightside and in the simplest terms might cancel after averaging all IMF directions.However, after averaging over all sampled IMFs, a coherent north-inward-south-outward pattern is still seen in Figures 1(d)-(f).Furthermore, we can compare two geographic green boxes when they are separately located on the dayside and nightside in Figures 1(e)-(f).The averaged B r /|B| over the WCR is more radial, i.e., darker red or blue, when the box is located on the nightside than on the dayside.That is, over the WCR, regardless of being connected to the surface or deeply draped, these open fields are not a result of ideal draping IMF, but a result of the Sun-Mars interaction that cannot be canceled by averaging IMFs, and thus is intrinsic by our definition.
In short, the observed large-scale dipole-like field might be a result of a combination of the MCR and WCR of crustal origin but the relative contribution of each component remains to be uncovered in future endeavors.

Concluding Remarks
In this study, we use magnetic topology to separate the induced and intrinsic components of Mars's hybrid magnetosphere: draped fields remain induced-like/Venus-like but the open fields reveal dipole-like intrinsic fields.The dipole-like intrinsic fields make Mars's solar wind interaction having an IMF clock angle bias, favoring the SIMF just like Earth, consistent with findings of a more open magnetosphere during ICME encounters (more N/S IMFs; Xu et al. 2018Xu et al. , 2019a)).This also has the implications of solar wind energy transfer to the Martian system, particularly during early Mars.The contribution from the dipole-like intrinsic fields can also help explain the tail twist.Overall, in terms of the solar wind interaction, previously most often compared to Venus, more and more research studies show that Mars is more Earth-like than previously thought, not just in the matter of localized processes near the strong crustal fields but also its global magnetospheric configuration.
illustrates the occurrence rates of the open topology at different altitude ranges (200-700 km) as a function of the IMF clock angle ( -B B tan z y 1 under the east IMF (A1)-(A3), the west IMF (B1)-(B3), and all IMFs (C1)-(C3).Panels (A1)-(B1) show a rotation of the tail current sheet from the expected orientation along Z MSO from an induced magnetosphere (perpendicular to the IMF), the same tail twists reported by previous studies (DiBraccio et al. 2018, 2022).With topology information available, we map the tail-lobe polarities separately for open and draped fields, as shown in panels (A2)-(B2) and (A3)-(B3), respectively.The rotation of the tail current sheet is mainly present in the open fields (A2)-(B2), up to 2 R M in the cylindrical axial distance ( ), the tail morphologies change significantly.In particular, for the case of the open topology (panel (C2)), the tail has a sunward polarity (B x /|B| > 0) in the north and a tailward (B x /|B| < 0) polarity in the south, with a tail current sheet along Y MSO .Note that the upstream IMF does not have a bias in any particular clock angles over the time range used in this study, as shown in Figure S1 in the supplementary material.In comparison, the tail for the draped topology (panel (C3)) shows an averaged Bx/|B| ∼ 0, except for small northsouth lobes at L MSO < 1 R M .This is because the contributions to the induced magnetotail lobes from predominantly east and west Parker IMFs cancel each other to the first approximation when combining all IMFs, resulting in near-zero B x /|B| in draped fields.In contrast, by isolating open fields, a dipole-like tail-lobe configuration is revealed, interpreted as the contribution from Mars's intrinsic fields.For all topologies combined (panel (C1)), a similar tail morphology of north-south lobes is seen but with smaller magnitudes.In general, the tail configuration of all topologies (panels (A1)-(C1)) is a combination of that of open and draped, as the tail is made up of ∼30% of open fields and ∼60% of draped fields (Xu et al. 2020).
shows the averaged B x /|B| for open fields (a)-(c) and draped fields (d)-(f) in the MSO Y-Z plane at X MSO = [0, 1], [−1, 0], and [−2, −1] R M , combining all IMF directions.All three panels for the open topology (a)-(c) show positive B x /|B| (sunward) in the north and negative B x /|B| (tailward) in the south for −2 < X MSO < 1 R M , resembling a magnetospheric configuration generated from a global dipole field interacting with upstream IMFs.In comparison, the magnetic polarities for draped fields (d)-(f) are close to zero, except for north-south lobes in the center at X MSO < 0 (panels (e)-(f)).

Figure 1 .
Figure 1.The occurrence rates of the open topology as a function of the IMF clock angle ( -B B tan z y 1 ) at different altitudes, (a) 200-300 km, (b) 300-500 km, (c) 500-700 km, red for "open-day," blue for "open-night," and black for all open fields.(d)-(f) The averaged local B r /|B| of open fields in geographic coordinates (with a bin size of 5°× 5°) as measured by MAVEN/MAG, limited to 200-300 km altitude, (d) for all subsolar geographic longitudes (SSL), (e) for southern strong crustal fields on the dayside (120°< SSL < 240°), and (f) for southern strong crustal fields on the nightside (SSL < 60°or SSL > 300°).The yellow dots and arrows in (e)-(f) indicate the ranges of SSL.The contours in (d)-(f) shows the radial component of modeled crustal magnetic fields at 250 km altitude (Langlais et al. 2019).

Figure 4 .
Figure 4. (A1)-(A2) Schematics show closed field lines from a southward-oriented dipole field (A1), which interacts with the IMF producing open field lines (A2).(B1)-(B4) Schematics show the tail field polarities (red for sunward and blue for tailward) from an intrinsic dipole-like field (B1) and the induced magnetotail comprised of draped IMFs (B2).(B3) Eastward IMF (dashed gray line) magnetically reconnects with open field lines (red and blue dashed lines) from quadrants 1 and 3, producing new open field lines (red and blue solid lines), overlaid over tail field polarities from the dipole-like field.(B4) The tail field polarities observed by MAVEN at X MSO = [−2, −1] R M for open fields under the east IMF condition.